In late January 2003, as Secretary of State Colin Powell prepared to argue the Bush administration's case against Iraq at the United Nations, veteran CIA officer Tyler Drumheller sat down with a classified draft of Powell's speech to look for errors. He found a whopper: a claim about mobile biological labs built by Iraq for germ warfare.

Drumheller instantly recognized the source, an Iraqi defector suspected of being mentally unstable and a liar. The CIA officer took his pen, he recounted in an interview, and crossed out the whole paragraph.

A few days later, the lines were back in the speech. Powell stood before the U.N. Security Council on Feb. 5 and said: "We have first-hand descriptions of biological weapons factories on wheels and on rails."

The sentence took Drumheller completely by surprise.

"We thought we had taken care of the problem," said the man who was the CIA's European operations chief before retiring last year, "but I turn on the television and there it was, again."

While the administration has repeatedly acknowledged intelligence failures over Iraqi weapons claims that led to war, new accounts by former insiders such as Drumheller shed light on one of the most spectacular failures of all: How U.S. intelligence agencies were eagerly drawn in by reports about a troubled defector's claims of secret germ factories in the Iraqi desert. The mobile labs were never found.

Drumheller, who is writing a book about his experiences, described in extensive interviews repeated attempts to alert top CIA officials to problems with the defector, code-named Curveball, in the days before the Powell speech. Other warnings came prior to President Bush's State of the Union address on Jan. 28, 2003. In the same speech that contained the now famous "16 words" on Iraqi attempts to acquire uranium, Bush spoke in far greater detail about mobile labs "designed to produce germ warfare agents."

The warnings triggered debates within the CIA but ultimately made no visible impact at the top, current and former intelligence officials said. In briefing Powell before his U.N. speech, George Tenet, then the CIA director, personally vouched for the accuracy of the mobile-lab claim, according to participants in the briefing. Tenet now says he did not learn of the problems with Curveball until much later and that he received no warnings from Drumheller or anyone else.

"No one mentioned Drumheller, or Curveball," Lawrence B. Wilkerson, Powell's chief of staff at the time, said in an interview. "I didn't know the name Curveball until months afterward."

Curveball's role in shaping U.S. declarations about Iraqi bioweapons capabilities was first described in a series of reports in the Los Angeles Times, and later in a March 2005 report by a presidential commission on U.S. intelligence failures regarding allegations that Iraq possessed weapons of mass destruction. But Drumheller's first-hand accounts add new detail about the CIA's embrace of a source whose credibility was already unraveling.

More than a year after Powell's speech, after an investigation that extended to three continents, the CIA acknowledged that Curveball was a con artist who drove a taxi in Iraq and spun his engineering knowledge into a fantastic but plausible tale about secret bioweapons factories on wheels.

But in the fall of 2002, Curveball was living the life of an important spy. A Baghdad native whose real name has never been released, he was residing in a safe house in Germany, where he had requested asylum three years earlier. In return for immigration permits for himself and his family, the Iraqi supplied Germany's foreign intelligence service with what appeared to be a rare insider's account of one of President Saddam Hussein's long-rumored WMD programs.

Curveball described himself as a chemical engineer who had worked inside an unusual kind of laboratory, one that was built on a trailer bed and produced weapons for germ warfare. He furnished detailed, technically complex descriptions of mobile labs and even described an industrial accident that he said killed a dozen people.

The German intelligence agency BND faithfully passed Curveball's stories to the Americans. Over time, the informant generated more than 100 intelligence reports on secret Iraqi weapons programs -- the only such reports from an informant claiming to have visited and worked in mobile labs. Other informants, also later discredited, had claimed indirect knowledge of mobile labs.

In late 2002, the Bush administration began scouring intelligence files for reports of Iraqi weapons threats. Drumheller was asked to press a counterpart from a European intelligence agency for direct access to Curveball. Other officials confirmed that it was the German intelligence service.

The German official declined but then offered a startlingly candid assessment, Drumheller recalled. "He said, 'I think the guy is a fabricator,' " Drumheller said, recounting the conversation with the official, whom he declined to name. "He said: 'We also think he has psychological problems. We could never validate his reports.' "

When Drumheller relayed the warning to his superiors in October 2002, it sparked what he described as "a series of the most contentious meetings I've ever seen" in three decades of government work.

Although no American had ever interviewed Curveball, analysts with the CIA's Center for Weapons Intelligence, Nonproliferation and Arms Control believed the informant's technical descriptions were too detailed to be fabrications.

"People were cursing. These guys were absolutely, violently committed to it," Drumheller said. "They would say to us, 'You're not scientists, you don't understand.' "

In January 2003, Drumheller received a new request from CIA headquarters to contact the German intelligence service about Curveball. This time, Drumheller recalled, the U.S. spy agency had three questions:

Could a U.S. official refer to Curveball's mobile lab accounts in an upcoming political speech?

Could the Germans guarantee that Curveball would stand by his account?

Could German intelligence verify Curveball's claims?

The reply from Berlin, as Drumheller recalls it, was less than encouraging: There are no guarantees.

"They said, 'We have never been able to verify his claims,' " Drumheller recalled. "And that was all sent up to Tenet's office."

When Drumheller listened to Bush's speech several days later, he was astonished to hear the mobile labs described in detail.

"Boom, there it was," he said.

A few days later, Drumheller was handed a draft of another key speech on Iraq: Powell's remarks to the U.N. Security Council accusing Hussein of reconstituting his WMD programs. This time, the speech included an obvious reference to Curveball -- an unnamed "chemical engineer" who worked in one of the labs -- as well as detailed drawings of mobile labs inspired by Curveball's descriptions.

Drumheller said he called the office of John E. McLaughlin, then the CIA deputy director, and was told to come there immediately. Drumheller said he sat across from McLaughlin and an aide in a small conference room and spelled out his concerns.

McLaughlin responded with alarm and said Curveball was "the only tangible source" for the mobile lab story, Drumheller recalled, adding that the deputy director promised to quickly investigate.

Portions of Drumheller's account of his meetings with McLaughlin and Tenet appear in the final report of the Silberman-Robb commission, which was appointed by Bush to investigate prewar U.S. intelligence failures on Iraq's weapons programs. The report cites e-mails and interviews with other CIA officials who were aware of the meetings.

In responding to questions about Drumheller, McLaughlin provided The Post with a copy of the statement he gave in response to the commission's report. The statement said he had no memories of the meeting with Drumheller and had no written documentation that the meeting took place.

"If someone had made these doubts clear to me, I would not have permitted the reporting to be used in Secretary Powell's speech," McLaughlin said in the statement.

In their briefings to Powell on Feb. 4, one day before the secretary's U.N. speech, Tenet and McLaughlin expressed nothing but confidence in the mobile-lab story, according to Wilkerson, Powell's chief of staff, who was present during the briefings.

"Powell and I were both suspicious because there were no pictures of the mobile labs," Wilkerson said. The drawings were constructed from Curveball's accounts.

But the CIA officials were persuasive. Wilkerson said the two men described the evidence on the mobile labs as exceptionally strong, based on multiple sources whose stories were independently corroborated.

On the eve of the U.N. speech, Drumheller received a late-night phone call from Tenet, who said he was checking final details of the speech. Drumheller said he brought up the mobile labs.

"I said: 'Hey, boss, you're not going to use that stuff in the speech . . . ? There are real problems with that,' " Drumheller said, recalling the conversation.

Drumheller recalled that Tenet seemed distracted and tired and told him not to worry.

The following day, Tenet was seated directly behind Powell at the U.N. Security Council as the secretary of state presented a detailed lecture and slide show about an Iraqi mobile biological weapons program.

Tenet, responding to questions about Drumheller's accounts, provided to The Post a statement he had given in response to the Silberman-Robb Commission report in which he said he didn't learn of the problems with Curveball until much later. He did not recall talking to Drumheller about Curveball, and said it was "simply wrong" for anyone to imply that he knew about the problems with Curveball's credibility.

"Nobody came forward to say there is a serious problem with Curveball or that we have been told by the foreign representative of the service handling him that there are worries that he is a 'fabricator,' " Tenet said in his statement.

In late summer 2003, seven months after the U.N. speech, Tenet called Powell to say that the Curveball story had fallen apart, Wilkerson said. The call amounted to an admission that all of the CIA's claims Powell used in his speech about Iraqi weapons were wrong.

"They had hung on for a long time, but finally Tenet called Powell to say, 'We don't have that one, either,' " Wilkerson recalled. "The mobile labs were the last thing to go."